The present invention generally relates to a disc inclination detecting apparatus, and in particular to a disc inclination detecting apparatus for detecting an inclination angle of an information recording disc upon recording or reproduction.
An optical disc is well known as an information recording disc. The optical disc is widely used as an information recording means of a high information recording density. However, the optical disc has difficulty in permanently maintaining its flat planes. In general, the relative height relationship between center and outer periphery parts of the disc varies, and the disc is thus distorted in the form of a dish. In particular, a disc with an air sandwiched type configuration is liable to have the inclination occurring at disc planes in inner and outer periphery parts having a spacer.
When recording information on a distorted disc, a spot of a light beam is deformed on an information recording surface of the disc, because the light beam cannot be perpendicularly projected thereon due to distortion of the disc. Therefore, there is the possibility that distorted pits are formed on the information recording surface and some pits are lacking. Deformation of the pits causes degradation of the frequency characteristic upon reproduction. The occurrence of inclination at the inner periphery part of the disc is a particularly serious problem where the disc is rotated so that an angular velocity of the disc is constant. On the other hand, upon reproduction, a beam of light which is emitted by a head or pickup and is reflected on the disc surface, does not return to the pickup accurately, so the reproduction of the recorded information is degraded.
In order to resolve the above problems, a disc inclination detecting apparatus has been proposed in the Japanese Patent Laid-Open No. 236,130/85, for example. This apparatus has a pair of a light-emitting element and a light-sensitive element which are located in the radial direction of the disc. A light beam emitted by the light-emitting element is reflected on the surface of the disc. A reflected light beam reaches the light-sensitive element, and moves on a light-sensitive surface of the light-sensitive element in a direction parallel to the axis of the disc, corresponding to the inclination of the disc. Then, the pickup is attitude-controlled by an attitude-controlling mechanism by the variation of the reflected light beam on the light-sensitive surface.
However, the conventional disc inclination detecting apparatus aforementioned has the following disadvantage. The reflected light beam moves on the light-sensitive surface in the direction parallel to the axis of the disc, even when the disc itself moves in the axial direction and correspondingly the distance between the surface of the disc and a pair of the light-emitting element and the light-sensitive element is varied. Therefore, the apparatus detects the movement of the disc in the axial direction as if the disc is inclined. It should be noted that this movement of the disc does not substantially affect the recording or reproducing operation.